Laminated glass with embrittled zone

ABSTRACT

A composite pane having at least one weak point for closing a vehicle opening, which includes at least one glass pane and at least one further pane, and an intermediate layer that connects the at least one glass pane and the at least one further pane to one another. The glass pane is provided with one or more weak points such that, in an event of an object striking the composite pane, the glass pane breaks at the one or more weak points, but the vehicle opening still remains closed at least by deforming the intermediate layer of the composite pane.

The invention relates to a composite pane with weak points for closing avehicle opening, according to the precharacterizing clause of Patentclaim 1. The invention likewise relates to the use of a composite paneaccording to the invention in a vehicle.

Widely differing embodiments of composite panes are used, mainly astransparent windows, in vehicles and buildings. The particular advantageof composite panes is regarded as being that they combine safetycharacteristics with a high rupture strength. The safety characteristicsare based on the fact that, when the composite panes are destroyed, thebroken-up individual panes are held together with the aid of thetear-resistant intermediate layer which connects the panes. Furthermore,no sharp edges project out of the composite pane. The rupture strengthof composite panes is in this case considerably greater than that ofindividual panes of the same thickness or of a prestressed individualpane. Prestressed panes also additionally have the disadvantage thatthey are sensitive to edge and surface damage and can break when suchdamage occurs. A further advantage of composite panes is that they canbe equipped with additional functional elements, which are arranged in aprotected form within the composite pane. This means that the functionalelements, for example transparent thin layers or electrical conductors,are arranged either in or on the intermediate layers, or on an internalpane surface.

The stated advantages relating to the rupture strength of compositepanes may have a disadvantageous effect if, in the event of anemergency, an area which is separated by a pane such as this has to beexited from or entered. Composite panes can be ruptured only withspecial tools.

In order to solve this problem, composite panes with weak points havebeen developed, which are used as emergency exit or entry systems.Composite safety glass panes with an emergency exit system are known,for example, from DE 44 28 690 A1 and from U.S. Pat. No. 5,350,613.Composite safety glass panes such as these are formed from at least tworigid panes with a polymer intermediate layer, with the intermediatelayer containing a weak point. The weak point described in DE 44 28 690A1 is represented by a local weakened area in the intermediate polymerlayer. This is achieved by reducing the adhesion of the layer to theglass, or else between different sheets of the layer.

German Patent DE 100 45 006 C1 describes a composite safety glass panewith a weak point, which contains prestressed glass panes and a polymerintermediate layer, which is interrupted at at least one point. In thiscase, the polymer intermediate layer is composed of two transparentplastics with different tear resistance, different tear expansion anddifferent further tearing resistance. As the weak point, the compositeglass pane contains the plastic with the lower tear resistance, thelower tear expansion and the lower further tearing resistance and, inthe region which does not represent the weak point, the plastic with thehigher tear resistance, the higher tear expansion and the higher furthertearing resistance. Furthermore, there are points in the composite glasspane without any polymer intermediate layer, at which grains of amaterial are provided whose hardness is higher than that of the glassthat is used. These grains are intended to penetrate into the surface ofthe prestressed glass pane when necessary, and destroy it.

When a composite pane is used as a windscreen or roof pane in a vehicle,its rupture resistance may be disadvantageous when a pedestrian, anin-line skater or a bicycle rider is thrown against the composite panein the event of an accident. Furthermore, windscreens normally haveexternal convex curvature. This shape results in a relatively highintrinsic strength, so that the glass does not break until higher forceshave been applied than in the case of planar panes or panes with concavecurvature. Since, in the event of an accident, a person is normallyfirst of all hit by the bumper bar or the engine bonnet on the lowerextremities and is thrown upwards, the head frequently strikes thewindscreen, which may result in very severe injuries.

Solutions are admittedly known for protection of people when vehiclescollide, but these relate to measures for providing energy-absorbentmaterials on the engine bonnet (DE 100 37 628 A1) or of raising theengine bonnet in the event of an accident with a person, in order toadvantageously influence the impact dynamics (DE 101 32 950 A1). Ameasure for protection of people which relates indirectly to thewindscreen is known from Laid-Open Specification DE 198 03 165 A1, whichdescribes a safety device for a motor vehicle in which an airbag coversa frame around the windscreen on the outer surface of the motor vehicleafter it has been unfolded.

The object of the present invention is to provide a composite pane forvehicles which provides better absorption for the energy of an objectstriking its surface.

According to the invention, this object is achieved by the features ofPatent claim 1. The features of the dependent claims indicateadvantageous developments of this subject matter.

The composite pane is thus a conventional laminate composed of a numberof rigid panes, which are connected to one another with the aid of atear-resistant intermediate layer which is used as an adhesive layer andis in each case arranged between them. At least one of the rigid panesis composed of glass which, according to the invention, is provided withone or more weak points. If a pane breaks as a result of an objectstriking the laminate, at least the glass pane then breaks while, incontrast, the intermediate layer remains intact as a cohesive layer orfilm. The weak points are arranged and are incorporated in the glasspane such that the breakage of the glass pane, and possibly of furtherpanes, does not allow the object striking it to pass through. Theopening which was closed by the composite pane is thus not opened up.The breakage of the glass pane and possibly of further panes results inthe stiffness of the composite pane being reduced. If the object is ahuman body, this leads to a considerable proportion of the impact energybeing absorbed when the person collides with the composite pane, orbeing dissipated by the breakage of the rigid pane and subsequentelastic or even plastic expansion of the intermediate layer. In certaincases, specifically when one of the further panes is composed of arelatively thin plastic, this plastic pane can likewise absorb impactenergy by elastic or plastic deformation. After the impact, theindividual panes are still connected to one another, so that on the onehand the vehicle opening in which the composite pane is used is stillclosed and, on the other hand, the human body is decelerated by thecomposite pane being more flexible as a result of the pane breaking.

This safety-net effect is admittedly also observed even withconventional composite glass panes, although the lack of weak pointsmeans that this is considerably more random than the effect which isachieved deliberately by the invention, and occurs at points that arenot locally defined.

Furthermore, these composite panes offer the resistance of theunweakened strength of the normal float glass panes against breakage.

A further aspect of the method of operation of the composite paneaccording to the invention is also that its edge remains firmlyconnected to the frame of the mounting opening. The composite pane doesnot penetrate completely into the vehicle interior, but is supported onthe said frame while the energy is being dissipated.

The weak points which have been mentioned are produced by weakening theglass pane, for example by means of mechanically stretching the glasspane. According to the invention, however, the weakening of the glasspane is preferably reduced by the introduction of a locally limitedstress gradient, that is to say of an inhomogeneous stress state in theglass, which deliberately reduces the fracture strength of the glass ina locally restricted area. The stress gradient is provided not only overthe thickness of the glass but also over the glass surface. If loadedbeyond a specific extent, for example as a result of bending of theglass pane, the strength in the area of the voltage gradient isexceeded, and the glass pane breaks.

A locally limited inhomogeneous stress state can be produced by applyinga stoving enamel and by stoving it at increased temperatures. If thecoefficient of expansion of the stoving enamel differs sufficiently fromthat of the glass pane, differences of more than 30% have been found tobe expedient, this results in sufficiently high stress inhomogeneitiesduring the cooling down of the glass/enamel composite, leading to thedesired effect. This method for production of the stress gradient ispreferred since it is associated with relatively little manufacturingcomplexity. Furthermore, composite panes for vehicles are frequentlyprovided with frame-like printing composed of stoving enamel, so thatthe stoving enamel to achieve the stress gradient and the stoving enamelfor the frame-like printing can be printed on in successive operations,and-can then be stoved jointly.

However, a locally limited stress gradient can also be introduced bylocal heating of the glass pane above the transformation temperature,and then by cooling it down rapidly. This method is widely used asthermal prestressing although, with the known methods, the entire paneis heated and cooled down homogeneously. Local heating can be achieved,for example, with the aid of a laser or of a gas flame.

In order to ensure that the broken glass pane does not penetrate throughthe intermediate layer after the pane has broken, that is to say thatthe laminate still closes the opening, the weak points can be arrangedoffset with respect to one another like a grid. This avoids the glasspane breaking along an elongated line, which could then form a cuttingedge and could cut through the intermediate layer. The distance betweenthe individual weak points and the size of the weak points may bebetween 0.1 mm and 200 mm depending on the nature and the form of theirarrangement.

If the weak points are arranged in the edge area of the composite pane,any optical adverse effects which may occur and are caused by theproduction of the stress gradient or by the different opticalcharacteristics of the areas with and without internal stresses arescarcely perceptible since they are located away from the main viewingarea. If required, weak points located in the edge area can be coveredwithout any major effort by the frame-like enamel there with which motorvehicle windows are frequently provided.

The weak points are preferably applied to that surface of the glass panewhich faces the interior. Since the curvature of the composite panepoints outwards, application of force from the interior of the vehiclewill not result in the glass pane breaking, because this produces acompressive stress close to the surface on that surface of the glasspane which faces the interior. As is known, compressive stresses can,however, be absorbed well by glass. When loaded from the inside, thecomposite pane according to the invention thus behaves scarcely anydifferently from a conventional composite pane. However, if the force isapplied from the outside, tensile stresses are produced close to thesurface on the surface of the glass pane facing the interior, which leadto the glass pane breaking as a result of the weakening produced by theweak points. However, the stress inhomogeneities should be designed suchthat forces which act on the installed (in particular bonded-in)composite pane as a result, for example, of bodywork deformation duringnormal operation cannot lead to the composite pane breaking.

The composite pane according to the invention is formed from a glasspane and one or more glass or plastic panes. The individual panes areconnected to one another with the aid of a tear-resistant intermediatelayer. The intermediate layer may, for example, be an adhesive filmcomposed of polyvinyl butyral or polyurethane. The intermediate layeritself may also be in the form of a laminate formed from a functionallayer and an adhesive layer.

1-13. (canceled)
 14. A composite pane having at lest one weak point forclosing a vehicle opening, comprising: at least one glass pane and atleast one further pane, and an intermediate layer which connects the atleast one glass pane and the at least one further pane to one another,wherein the glass pane is provided with one or more weak points suchthat, in an event of an object striking the composite pane, the glasspane breaks at the one or more weak panes, but the vehicle opening stillremains closed at least by deforming the intermediate layer of thecomposite panel.
 15. A composite pane according to claim 14, wherein theone or more weak points are locally limited inhomogeneous stress statesintroduced into the glass pane.
 16. A composite pane according to claim15, wherein the inhomogeneous stressed states are achieved by printingon and then stoving a stoving enamel.
 17. A composite pane according toclaim 15, wherein the inhomogeneous stressed states are produced bylocal thermal prestressing.
 18. A composite pane according to claim 17,wherein the thermal prestressing is produced by local heating with a gasflame followed by sudden cooling down.
 19. A composite pane according toclaim 17, wherein the thermal prestressing is produced by local heatingwith a laser, followed by sudden cooling down.
 20. A composite paneaccording to claim 14, wherein the one or more weak points are arrangedoffset with respect to one another in a grid.
 21. A composite paneaccording to claim 14, wherein the one or more weak points are arrangedin an edge area of the composite pane.
 22. A composite pane according toclaim 14, wherein the at least one further pane is composed of glass.23. A composite pane according to claim 22, wherein one or more weakpoints are also introduced into the at least one further glass pane. 24.A composite pane according to claim 14, wherein the intermediate layerwhich connects the at least one glass pane and the at least one furtherpane to one another comprises a thermoplastic adhesive sheet.
 25. Acomposite pane according to claim 14, wherein the one or more weakpoints are arranged on a side of the glass pane that faces away from aside on which the object strikes.
 26. Use of a composite pane accordingto claim 14 as a windscreen or roof pane in a vehicle.